The use of dielectric coatings on polished metallic or other reflective surfaces to improve the reflectivity is well known. In U.S. Pat. No. 2,519,722, for example, there is described such an arrangement in which superimposed light transmitting layers of nonmetallic material are used having alternately low and high indices of refraction. The thickness of these films is held to a quarter wavelength, within fairly close tolerances to achieve reflectivities of the order of 99% or better. Light reflected from a mirror, even a coated mirror of the type described in the above-identified patent, experiences a substantially zero phase change between the .rho. polarization and the s polarization components. However, for certain applications it is desirable to obtain a 90.degree. phase change between the s and p polarization components. For example, such a reflective surface functions in equivalent manner to a phase retarder, such as a quarterwave plate, to convert linearly polarized light into elliptical or circularly polarized light (or vice versa). Quarterwave plates require propagation of the light through the material and they are very frequency sensitive so as to be substantially monochromatic in operation. Other applications where a 90.degree. phase change between the s and p polarization components is desirable is in conical mirrors, which function as double-bounce inverting devices. Such conical reflectors with their double-bounce reflection do not preserve the linear polarization state of an incident beam. However, by providing a 90.degree. phase shift at each reflective surface, this problem is overcome, since the angle of incidence of each reflection is the same.